Force and acceleration sensor using four wave mixing technique

ABSTRACT

A sensor for measuring force, the sensor including: a light source; and a mixing medium in optical communication with the light source and exposed to the force; wherein four wave mixing of light interacting with the mixing medium provides a signal that indicates the force.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation in part application of U.S. Ser. No.11/933,512, filed on Nov. 1, 2007, the contents of which areincorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention disclosed herein relates to a sensor for measuring atleast one of temperature, pressure and force. In particular, the sensoris used with a logging instrument in a borehole.

2. Description of the Related Art

In exploration for hydrocarbons, it is important to make accuratemeasurements of various properties of geologic formations. Inparticular, it is important to determine the various properties with ahigh degree of accuracy so that drilling resources are used efficiently.

Generally, oil and gas are accessed by drilling boreholes into thesubsurface of the earth. The boreholes also provide access for takingmeasurements of the geologic formations.

Well logging is a technique used to take measurements of the geologicformations from the boreholes. Well logging can also be used to takemeasurements of conditions in the boreholes. The conditions in theboreholes are important to know to safely and efficiently use drillingresources.

In one embodiment, a “logging instrument” is lowered on the end of awireline into a borehole. The logging instrument sends data via thewireline to the surface for recording. Output from the logginginstrument comes in various forms and may be referred to as a “log.”Many types of measurements are made to obtain information about thegeologic formations and conditions in the borehole. Two important logsare a temperature log and a pressure log.

The temperature log records temperature in the borehole at variousdepths. The temperature log can provide indication of temperaturegradients in the borehole. The temperature log can be compared to areference temperature log. Departures from the reference temperature logcan indicate entry of fluids into the borehole. Conversely, thedepartures can indicate fluids exiting the borehole. In addition, thetemperature log can be used to detect leaks in a borehole casing orleaks from a valve.

The pressure log records pressure at various depths within the borehole.Accurate pressure measurements can be used to monitor depletion ofreservoirs associated with the production of hydrocarbons. Further,accurate measurements of pressure in the borehole are needed duringdrilling operations. It is important to monitor pressure during drillingoperations to keep the pressure under control. If the pressure is notkept under control, then an uncontrolled release of oil and gas to thesurface (known as a “blowout”) can result. The blowout can causepersonal injuries, drilling rig damage, environmental damage, and damageto underground reservoirs.

Another important measurement for analyzing a formation is that of theforce of gravity or gravitational acceleration. For example, changes ingravitational acceleration can be related to depletion of hydrocarbonsin a reservoir. In addition, absolute gravitational acceleration orchanges in gravitational acceleration can be related to the truevertical depth (TVD) of the logging instrument in the borehole.

Measuring the TVD is especially important in a borehole deviated fromthe vertical. Logging data is generally correlated to the TVD at whichthe logging data was obtained. It is important for the TVD to beaccurate or the logging data can be corrupted. In a deviated borehole,the length of the wireline cannot be relied on to provide an accurateindication of the true vertical depth of the logging instrument becausesome of the wireline will be in a horizontal orientation. Even in anon-deviated borehole, the wireline can be subject to stretching therebyleading to an inaccurate measurement of the TVD.

In addition to measuring pressure, temperature, and gravitationalacceleration, it is also important to be able to measure force such as aforce of acceleration or a force imposed on a static object. Bymeasuring force, motions and stresses related to a drill string can bemonitored.

Therefore, what are needed are techniques to measure temperature,pressure, and force within a borehole. In particular, the techniquesprovide for high accuracy measurements.

BRIEF SUMMARY OF THE INVENTION

Disclosed is an embodiment of a sensor for measuring force, the sensorincluding: a light source; and a mixing medium in optical communicationwith the light source and exposed to the force; wherein four wave mixingof light interacting with the mixing medium provides a signal thatindicates the force.

Also disclosed is one example of a method for measuring force, themethod including: exposing a mixing medium to the force; illuminatingthe mixing medium with at least two beams of light, wherein the lightinteracts with the mixing medium by four wave mixing of the light;measuring a characteristic of light emitted from the mixing medium as aresult of the four wave mixing; and determining the force from themeasured characteristic.

Further disclosed is an embodiment of a system for measuring force in aborehole, the system including: a logging instrument configured to beconveyed through the borehole, the logging instrument having: a lightsource; a mixing medium in optical communication with the light sourceand exposed to the force, wherein four wave mixing of light interactingwith the mixing medium provides light with a characteristic thatindicates the force; and a light detector in optical communication withthe mixing medium and configured to measure the characteristic.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter, which is regarded as the invention, is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings, wherein like elements arenumbered alike, in which:

FIGS. 1A, 1B, and 1C, collectively referred to as FIG. 1, illustrateaspects of four way mixing using three examples;

FIG. 2 illustrates an exemplary embodiment of a logging instrument in aborehole penetrating the earth;

FIG. 3 illustrates an exemplary embodiment of a sensor for measuring atleast one of temperature and pressure;

FIG. 4 illustrates a graph of wavelength of an anti-Stokes wave emittedfrom a mixing medium versus temperature of the mixing medium;

FIG. 5 illustrates an exemplary embodiment of a computer coupled to thelogging instrument;

FIG. 6 presents one example of a method for measuring at least one oftemperature, pressure and force within the borehole; and

FIG. 7 illustrates aspects of a proof mass coupled to the mixing mediumfor measuring gravitational force.

DETAILED DESCRIPTION OF THE INVENTION

Disclosed are embodiments of techniques to measure at least one oftemperature, pressure and force with high accuracy. The techniquesinclude a sensor that is sensitive to temperature, pressure and force.The sensor interacts at least two inputs of light in a mixing medium.The inputs of light interact using four wave mixing (also referred to as“four photon mixing”). As a result of the four wave mixing, an output oflight will be emitted from the mixing medium that can include two beamsof light. At least one of intensity and wavelength of light of each ofthe beams can be correlated to at least one of the temperature, pressureand force experienced by the mixing medium.

For convenience, certain definitions are provided. The term “four wavemixing” relates to an interaction between at least two input lightwaves. The interaction can result in producing two output light waves, a“Stokes wave” and an “anti-Stokes wave.” Each of the Stokes wave and theanti-Stokes wave generally has a different wavelength from thewavelength of the input light waves. However, the sum of the momentum ofeach of the Stokes wave and the anti-Stokes wave equals the sum of themomentum of each of the input light waves. In some embodiments, theoutput of light from the mixing medium can include more than two beamsof light. In these embodiments, the sum of the momentum of each of theinput light waves equals the sum of the momentum of each light wave inthe output of light. Because the Stokes wave results from combining theinput light waves in phase and the anti-Stokes wave results fromcombining the input waves out of phase, the intensity of the Stokes wavecan be as much as ten thousand greater than the anti-Stokes wave.

FIG. 1 illustrates aspects of four wave mixing using three examples. Forillustration purposes, only one output light wave of frequency 2f₁-f₂ isdepicted. Other output light waves of different frequencies can also beproduced using four wave mixing depicted in these examples. Referring toFIG. 1A, two input light waves of frequency f₁ and one input light waveof frequency f₂ interact within a mixing medium 26 to produce the outputlight wave of frequency 2f₁-f₂. Referring to FIG. 1B, one input lightwave of frequency f₁ and one input light wave of frequency f₂ interactwithin the mixing medium 26 to produce an intermediate light wave offrequency f₁-f₂. The intermediate light wave then interacts within themixing medium 26 with one input light wave of frequency f₁ to producethe output light wave of frequency 2f₁-f₂. Referring to FIG. 1C, twoinput light waves each with frequency f₁ interact within the mixingmedium 26 to produce an intermediate light wave of frequency 2f₁. Theintermediate light wave of frequency 2f₁ then interacts within themixing medium 26 with one input light wave with frequency f₂ to producethe output light wave of frequency 2f₁-f₂.

In some instances, the two input light waves of frequency f₁ (asdepicted in FIGS. 1A and 1C) may be provided by one beam of light wherea portion of the photons may be considered as one input light wave andthe remainder of photons may be considered as the other input lightwave. Each portion of the photons may also be referred to as a beam oflight.

The term “overlap” relates to the requirement that the at least twoinput light waves must generally occupy the same space at the same timefor the four wave mixing to occur. The term “susceptibility” relates toa measure of how easily a dielectric material polarizes in response toan electric field. The term “mixing medium” relates to a material thatmediates or enables wave mixing via the second order electricsusceptibility (χ⁽²⁾) and the third order electric susceptibility (χ⁽³⁾)of the mixing medium. In one embodiment, the mixing medium can be abirefringent material. The birefringent material includes a “fast axis”and a “slow axis.” Light polarized along the fast axis will travelthrough the birefringent material faster than light polarized along theslow axis. The term “phase matching” relates to the process of selectingdirections of polarization and frequencies of the input light waves inorder to maintain a constant phase relationship between all the lightwaves in the mixing medium. Maintaining a constant phase relationshipavoids destructive interference, which can interfere with the four wavemixing.

Phase matching can be described for a birefringent optical fiber used asthe mixing medium. The phase matching condition is based on the sum ofthe wave vectors of the output light waves equaling the sum of the wavevectors of the input light waves. Equation (1) mathematically describesa phase matching condition resulting from a change in temperature ΔTwhere Δk(Δ ν) represents the phase mismatch with a frequency shift of (Δν) for the mixing medium 26 that is the birefringent optical fiber andΔf(Δ ν) is determined using equations (2) through (6). Equation (1)applies when the polarization of the input (or “pump”) light wave isalong the slow axis of the birefringent optical fiber and thepolarizations of the Stokes and the anti-Stokes waves are along the fastaxis of the birefringent optical fiber.Δk(Δ ν)−Δf( ν)=0  (1)Δf(Δ ν)=−2(B _(S) +B _(G))(2π ν _(p))  (2)

where B_(S) represents stress induced birefringence, B_(G) representsgeometrical anisotropy birefringence, and ν _(p) represents thenormalized frequency (ν_(p)/speed of light in free space) of an input(or pump) light wave.B _(S) =B _(S0) H(V)  (3)

where H(V) represents the stress difference in the mixing medium 26 atnormalized frequency V and B_(S0) represents the residual stress inducedbirefringence of the mixing medium 26.B _(G) =nε ² ΔG(V)  (4)

where n represents the refractive index at the core center of theoptical fiber, ε represents the ellipticity of the optical fiber, andG(V) represents normalized phase constant difference at normalizedfrequency V.B _(S0)=−(1/2)n ³(P ₁₁ −P ₁₂)(α₀−α₁)qΔT  (5)

where P₁₁, and P₁₂ are strain coefficients of silica used to make theoptical fiber, α₀ and α₁ are thermal coefficients of expansion of thesilica and dopant used to make the optical fiber, and q represents aproportionality constant.ε=1−(a _(x) /a _(y))  (6)

where a_(x) and a_(y) are coefficients that describe the ellipticity ofthe optical fiber.

The frequency shift Δ ν resulting from phase matching can be correlatedto the stress or physical change imposed upon the mixing medium 26 fromthe change in at least one of temperature ΔT, pressure ΔP and force ΔF.While the teachings discuss determining temperature, pressure or forceimposed upon the mixing medium 26, in some embodiments a change intemperature, pressure or force may be measured and then referenced to areference temperature, pressure or force, respectively, to determine thetemperature, pressure or force.

The term “housing” relates to a structure of a logging instrument. Thehousing may be used to at least one of contain and support a device usedwith the logging instrument. The device can be the sensor describedabove. The sensor is sized to fit within the housing of a logginginstrument.

Referring to FIG. 2, one embodiment of a well logging instrument 10 isshown disposed in a borehole 2. The logging instrument 10 can be usedfor measuring at least one of temperature, pressure and force. Thelogging instrument 10 includes an instrument housing 8 adapted for usein the borehole 2. The borehole 2 is drilled through earth 7 andpenetrates formations 4, which include various formation layers 4A-4E.The logging instrument 10 is generally lowered into and withdrawn fromthe borehole 2 by use of an armored electrical cable 6 or similarconveyance as is known in the art. In the embodiment of FIG. 1, a sensor3, used for measuring at least one of temperature, pressure and force,is shown disposed within the housing 8. The sensor 3 is coupled to anelectronic unit 9 that at least one of records and processes signalsreceived from the sensor 3.

In some embodiments, the borehole 2 includes materials such as would befound in oil exploration, including a mixture of liquids such as water,drilling fluid, mud, oil and formation fluids that are indigenous to thevarious formations. One skilled in the art will recognize that thevarious features as may be encountered in a subsurface environment maybe referred to as “formations.” Accordingly, it should be consideredthat while the term “formation” generally refers to geologic formationsof interest, that the term “formations,” as used herein, may, in someinstances, include any geologic points of interest (such as a surveyarea).

For the purposes of this discussion, it is assumed that the borehole 2is vertical and that the formations 4 are horizontal. The teachingsherein, however, can be applied equally well in deviated or horizontalwells or with the formation layers 4A-4E at any arbitrary angle. Theteachings are equally suited for use in logging while drilling (LWD)applications, measurement while drilling (MWD) and in open-borehole andcased-borehole wireline applications. In LWD/MWD applications, thelogging instrument 10 may be disposed in a drilling collar and conveyedby a drill string in the borehole 2. When used in LWD/MWD applications,drilling may be halted temporarily to prevent vibrations while thesensor 3 is used to perform a measurement of at least one oftemperature, pressure and force. In addition, the logging instrument 10may be configured for being conveyed by slickline or by coiled tubing asis known in the art.

FIG. 3 illustrates an exemplary embodiment of the sensor 3. Referring toFIG. 3, the sensor 3 includes three light sources, a first light source21, a second light source 22 and a third light source 23. In general,the wavelength of light emitted from the second light source 22, λ₂, isclose to the wavelength of light emitted from the first light source 21,λ₁, but not the same. Also, in general, the wavelength of light emittedfrom the third light source 23, λ₃, is close to λ₁ and λ₂ but differentfrom both λ₁ and λ₂ by an amount slightly greater than the absolutevalue of the difference between λ₁ and λ₂.

An embodiment of any of the light sources is a laser. Another embodimentof any of the light sources may include a broadband light source. Whenthe broadband light source is used, an optical filter may be used toprovide certain wavelengths of light to the mixing medium 26. Theoptical filter may include at least one of a fiber Bragg grating and aFabry-Perot cavity.

Referring to FIG. 3, light from the three light sources is superimposedby a beam combiner 24. As shown in FIG. 3, a combined light beam 25 isemitted from the beam combiner 24. The combined light beam 25 enters themixing medium 26 where the four wave mixing occurs. In the embodiment ofFIG. 3, the four wave mixing results in two light beams being emittedfrom the mixing medium 26, a Stokes wave 27 and an anti-Stokes wave 28.

Referring to FIG. 3, a grating 20 spatially separates the Stokes wave 27and the anti-Stokes wave 28 to aid in measuring characteristics of eachof the Stokes wave 27 and the anti-Stokes wave 28. In general, thegrating 20 is optically coupled to the mixing medium 26. Components inthe sensor 3, such as the beam combiner 24 and the grating 20 forexample, are generally selected to conserve polarization of lightentering the components.

The properties of each of the Stokes wave 27 and the anti-Stokes wave 28are related to an amount of overlap experienced by the light beamsemitted from the first light source 21, the second light source 22 andthe third light source 23 in the mixing medium 26. In turn, the amountof overlap can be related to an amount of birefringence or a nonlinearoptical property exhibited by the mixing medium 26. The amount ofbirefringence or the nonlinear optical property in the mixing medium 26can be changed by physically changing the mixing medium 26. Examples ofphysical change to the mixing medium 26 include at least one ofmechanical expansion, mechanical contraction, and physical deformationof a shape of the mixing medium 26. Physical changes to the mixingmedium 26 may be accomplished by changing at least one of thetemperature, the pressure and the force exerted on the mixing medium 26.For example, for the embodiment of the mixing medium 26 as abirefringent optical fiber, at least one of mechanically expanding,mechanically contracting, and mechanically deforming the shape theoptical fiber will change the birefringence of the optical fiber.Referring to FIG. 3, at least one of temperature, pressure, and forcecausing a physical change to the mixing medium 26 is represented byarrow 19.

An embodiment of the mixing medium 26 can include an optical fiberexhibiting birefringence resulting from exposure to at least one oftemperature, pressure and force. In one embodiment, the optical fibermay be made from fused silica. Another embodiment of the mixing medium26 includes a birefringent crystal. Still another embodiment of themixing medium 26 includes periodically poled lithium niobate (PPLN).PPLN is a crystal having nonlinear optical properties that are strongerthan the nonlinear properties in conventional materials. A polingprocess to produce PPLN inverts the crystal structure generally everyfew microns along the crystal. The poling process includes applying anintense electric field to the crystal to rearrange the crystal structurepermanently at an atomic level. Still another embodiment of the mixingmedium 26 includes a gallium-arsenide (GaAs)/aluminum-gallium-arsenide(AlGaAs) semiconductor structure, which also has nonlinear opticalproperties. In one embodiment, the GaAs/AlGaAs semiconductor structureincludes adjacent optical paths, one path of GaAs and one path ofAlGaAs. Four wave mixing of light occurs along the interface of thepaths. Physical change of the PPLN or the GaAs/AlGaAs semiconductorstructure will cause a change in the associated nonlinear opticalproperties.

Characteristics of each of the Stokes wave 27 and the anti-Stokes wave28 include an intensity and a wavelength. The wavelength may be apredominant wavelength among a range of wavelengths. At least one of theintensity and the wavelength of each of the Stokes wave 27 and theanti-Stokes wave 28 may be correlated to at least one of thetemperature, the pressure and the force experienced by the mixing medium26.

As shown in FIG. 3, a light detector 29 is used to measure thecharacteristics of at least one the Stokes wave 27 and the anti-Stokeswave 28. The characteristics may include at least one of intensity andwavelength. In one embodiment, the light detector 29 is an opticalspectrum analyzer used to measure a wavelength of light. In anotherembodiment, the light detector 29 is at least one of a photomultipliertube and a photodiode used for measuring an intensity of light.

In general, the mixing medium 26 will require a calibration to at leastone of temperature, pressure and force. The calibration can includevarying a property to be measured (at least one of temperature, pressureand force) and measuring at least one of intensity and wavelength foreach of the Stokes wave 27 and the anti-Stokes wave 28 emitted by themixing medium 26. FIG. 4 is one example of a calibration curve of themixing medium 26 for measuring temperature. As shown in FIG. 4, as thetemperature of the mixing medium 26 increases, the wavelength of theanti-Stokes wave 28 increases. A similar curve can be developed for theStokes wave 27.

Because the birefringence of the mixing medium 26 can be related to atemperature and a pressure together experienced by the mixing medium 26,two sensors 3 can be used to compensate for one of temperature andpressure. If in one embodiment a temperature is to be measured, then onesensor 3 (first sensor 3) can be exposed to a temperature to be measuredand an ambient pressure. The other sensor 3 (second sensor 3) can beexposed to a constant temperature and the same ambient pressure that isexerted upon the first sensor 3. In this embodiment, the temperature canbe measured while compensating for pressure effects. Similarly, twosensors 3 can be used to measure pressure and compensate for temperatureeffects. If in one embodiment pressure is to be measured, then the firstsensor 3 can be exposed to the pressure to be measured and an ambienttemperature. The second sensor 3 can be exposed to a constant pressureand the same ambient temperature to which the first sensor 3 is exposed.Therefore, the pressure can be measured while compensating for thetemperature effects. The compensating may include subtracting thecharacteristics of the output of light from the mixing medium 26associated with the second sensor 3 from the characteristics of theoutput of light emitted from the mixing medium 26 associated with thefirst sensor 3. Similar to the temperature and pressure compensationdiscussed above, at least one of ambient temperature and ambientpressure can be compensated for when measuring force.

In order to compensate for an ambient parameter, such as temperature orpressure, the first sensor 3 can be exposed to the force being measuredand the ambient parameter. The second sensor 3 can then be exposed tothe force being measured and the parameter held constant. Thus, theoutput of the first sensor 3 can be compensated by the output of thesecond sensor 3. As in the preceding paragraph, the compensating caninclude subtracting the output of the second sensor 3 from the output ofthe first sensor 3.

In some embodiments, adjustments to the light emitted from the inputlight sources may be necessary for phase matching. One of theadjustments may include varying the wavelength of light emitted from atleast one input light source. Another adjustment may include changingthe polarization of the light emitted from at least one light source.The polarization may be changed with respect to the orientation of thefast axis and the slow axis of a birefringent material used for themixing medium 26. The teachings include components, such as an inputlight source with a variable wavelength of output light, used for makingthe adjustments.

Generally, the well logging instrument 10 includes adaptations as may benecessary to provide for operation during drilling or after a drillingprocess has been completed.

Referring to FIG. 5, an apparatus for implementing the teachings hereinis depicted. In FIG. 5, the apparatus includes a computer 50 coupled tothe well logging instrument 10. Typically, the computer 50 includescomponents as necessary to provide for the real time processing of datafrom the well logging instrument 10. Exemplary components include,without limitation, at least one processor, storage, memory, inputdevices, output devices and the like. As these components are known tothose skilled in the art, these are not depicted in any detail herein.

Generally, some of the teachings herein are reduced to an algorithm thatis stored on machine-readable media. The algorithm is implemented by thecomputer 50 and provides operators with desired output. The output istypically generated on a real-time basis.

The logging instrument 10 may be used to provide real-time measurementsof at least one of temperature and pressure. As used herein, generationof data in “real-time” is taken to mean generation of data at a ratethat is useful or adequate for making decisions during or concurrentwith processes such as production, experimentation, verification, andother types of surveys or uses as may be opted for by a user oroperator. Accordingly, it should be recognized that “real-time” is to betaken in context, and does not necessarily indicate the instantaneousdetermination of data, or make any other suggestions about the temporalfrequency of data collection and determination.

A high degree of quality control over the data may be realized duringimplementation of the teachings herein. For example, quality control maybe achieved through known techniques of iterative processing and datacomparison. Accordingly, it is contemplated that additional correctionfactors and other aspects for real-time processing may be used.Advantageously, the user may apply a desired quality control toleranceto the data, and thus draw a balance between rapidity of determinationof the data and a degree of quality in the data.

FIG. 6 presents one example of a method 60 for performing a measurementof at least one of temperature, pressure, and force in the borehole 2.The method 60 calls for (step 61) exposing the mixing medium 26 to atleast one of the temperature, the pressure and the force. Further, themethod 60 calls for (step 62) illuminating the mixing medium 26 with atleast two beams of light, wherein the light interacts with the mixingmedium 26 by four wave mixing of the light. Further, the method 60 callsfor (step 63) measuring a characteristic of light emitted from themixing medium 26 as a result of the four wave mixing. Further, themethod 60 calls for (step 64) determining the at least one of thetemperature, the pressure and the force from the characteristic.

FIG. 7 depicts aspects of using the sensor 3 to measure gravitationalforce and thereby determining gravitational acceleration. Referring toFIG. 7A, a proof mass 70 may be coupled to the mixing medium 26. Themass of the proof mass 70 is selected so that a change in magnitude ofgravitational force 71 acting upon the proof mass 70 physically changesthe mixing medium 26. FIG. 7B depicts an increase in the magnitude ofthe gravitational force 71 acting upon the proof mass 70 causing aphysical change in the proof mass 70. The physical change in turn causesa change in the birefringence of the mixing medium 26. Thus, a change ina characteristic of light emitted from the mixing medium 26 can then berelated to a change in magnitude of the gravitational force 71.

In certain embodiments, a string of two or more logging instruments 10may be used where each logging instrument 10 includes at least onesensor 3. In these embodiments, a response from each logging instrument10 may be used separately or combined with other responses to form acomposite response.

In support of the teachings herein, various analysis components may beused, including digital and/or analog systems. The digital and/or analogsystems may be used in the electronic unit 9 for at least one ofrecording and processing signals from the sensor 3. The electronic unit9 may be disposed at least one of in the logging instrument 10 and atthe surface of the earth 7 as part of the computer 50. The system mayhave components such as a processor, storage media, memory, input,output, communications link (wired, wireless, pulsed mud, optical orother), user interfaces, software programs, signal processors (digitalor analog) and other such components (such as resistors, capacitors,inductors and others) to provide for operation and analyses of theapparatus and methods disclosed herein in any of several mannerswell-appreciated in the art. It is considered that these teachings maybe, but need not be, implemented in conjunction with a set of computerexecutable instructions stored on a computer readable medium, includingmemory (ROMs, RAMs), optical (CD-ROMs), or magnetic (disks, harddrives), or any other type that when executed causes a computer toimplement the method of the present invention. These instructions mayprovide for equipment operation, control, data collection and analysisand other functions deemed relevant by a system designer, owner, user orother such personnel, in addition to the functions described in thisdisclosure.

Further, various other components may be included and called upon forproviding for aspects of the teachings herein. For example, a powersupply (e.g., at least one of a generator, a remote supply and abattery), cooling component, heating component, pressure retaining ortransmitting component, insulation, sensor, transmitter, receiver,transceiver, antenna, controller, lens, optical unit, optical filter,light source, light detector, electrical unit or electromechanical unitmay be included in support of the various aspects discussed herein or insupport of other functions beyond this disclosure.

Elements of the embodiments have been introduced with either thearticles “a” or “an.” The articles are intended to mean that there areone or more of the elements. The terms “including” and “having” areintended to be inclusive such that there may be additional elementsother than the elements listed. The conjunction “or” when used with alist of at least two terms is intended to mean any term or combinationof terms. The terms “first” and “second” are used to distinguishelements and are not used to denote a particular order.

It will be recognized that the various components or technologies mayprovide certain necessary or beneficial functionality or features.Accordingly, these functions and features as may be needed in support ofthe appended claims and variations thereof, are recognized as beinginherently included as a part of the teachings herein and a part of theinvention disclosed.

While the invention has been described with reference to exemplaryembodiments, it will be understood that various changes may be made andequivalents may be substituted for elements thereof without departingfrom the scope of the invention. In addition, many modifications will beappreciated to adapt a particular instrument, situation or material tothe teachings of the invention without departing from the essentialscope thereof. Therefore, it is intended that the invention not belimited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A sensor for measuring force, the sensor comprising: a light source;and a mixing medium in optical communication with the light source andexposed to the force; wherein four wave mixing of light interacting withthe mixing medium provides a signal that indicates the force.
 2. Thesensor as in claim 1, further comprising at least one additional lightsource configured to illuminate the mixing medium.
 3. The sensor as inclaim 1, wherein the light source comprises at least one of a laser anda broadband light source coupled to an optical filter.
 4. The sensor asin claim 1, wherein the light source is configured to vary a wavelengthof light emitted from the light source.
 5. The sensor as in claim 1,further comprising a beam combiner optically coupled to the mixingmedium and to the light source.
 6. The sensor as in claim 1, furthercomprising a light detector optically coupled to the mixing medium andconfigured to detect light emitted from the mixing medium.
 7. The sensoras in claim 6, wherein the light detector comprises at least one of anoptical spectrum analyzer, a photomultiplier tube and a photodiode. 8.The sensor as in claim 1, wherein the mixing medium comprises abirefringent material.
 9. The sensor as in claim 8, wherein thebirefringent material comprises at least one of an optical fiber and acrystal.
 10. The sensor as in claim 1, wherein the mixing mediumcomprises periodically poled lithium niobate.
 11. The sensor as in claim1, wherein the mixing medium comprises a gallium-arsenide(GaAs)/aluminum-gallium-arsenide (AlGaAs) semiconductor structure. 12.The sensor as in claim 1, further comprising a grating optically coupledto the mixing medium, the grating configured to spatially separate lightemitted from the mixing medium.
 13. The sensor as in claim 1, furthercomprising a proof mass coupled to the mixing medium wherein the proofmass is configured to physically change the mixing medium in response toa change in gravitational force.
 14. A method for measuring force, themethod comprising: exposing a mixing medium to the force; illuminatingthe mixing medium with at least two beams of light, wherein the lightinteracts with the mixing medium by four wave mixing of the light;measuring a characteristic of light emitted from the mixing medium as aresult of the four wave mixing; and determining the force from themeasured characteristic.
 15. The method as in claim 14, wherein thecharacteristic comprises at least one of intensity and wavelength of aStokes wave.
 16. The method as in claim 14, wherein the characteristiccomprises at least one of intensity and wavelength of an anti-Stokeswave.
 17. The method as in claim 14, further comprising: exposing themixing medium to at least one of temperature and pressure; anddetermining the at least one of temperature and pressure from themeasured characteristic.
 18. The method as in claim 14, wherein theforce is gravitational force.
 19. The method as in claim 14, wherein themethod is implemented by instructions stored on machine-readable media.20. A system for measuring force in a borehole, the system comprising: alogging instrument configured to be conveyed through the borehole, thelogging instrument comprising: a light source; a mixing medium inoptical communication with the light source and exposed to the force,wherein four wave mixing of light interacting with the mixing mediumprovides light with a characteristic that indicates the force; and alight detector in optical communication with the mixing medium andconfigured to measure the characteristic.